Technical Field
The present disclosure relates to a method of detecting an object by means of a detection signal supplied by a proximity sensor.
The present disclosure relates in particular to object detectors comprising a proximity sensor of the capacitive type.
Description of the Related Art
FIG. 1 schematically shows a conventional proximity detector DTC1. Detector DTC1 comprises a proximity sensor controller 10, a sensitive portion 11, and a signal processing unit SPU1. Sensor controller 10 comprises electronic means of controlling and of reading sensitive portion 11, supplying a detection signal Sd. Signal Sd has a value, for example its amplitude, that varies as a function of the distance separating an object 12 from sensitive portion 11. The value of signal Sd also evolves depending on various environmental parameters such as the temperature, the dielectric constant of air which is a function of the ambient humidity, the proximity of objects other than the detected object, etc.
Unit SPU1 ensures the processing of signal Sd and supplies a state signal ST having two values DET and NDET, respectively signifying “object detected” or “object not detected”.
A conventional processing method of signal Sd as executed by unit SPU1 is shown in FIGS. 2A, 2B. It is supposed here that signal Sd has a value that increases as the object approaches sensor 10. The method comprises the following steps:                unit SPU1 calculates a reference signal Sr, the value of which varies more slowly than that of signal Sd, for example by low-pass filtering of signal Sd,        unit SPU1 defines a detection threshold Th1 greater than reference signal Sr, for example by adding an offset OF1 to the value of reference signal Sr,        when the value of signal Sd becomes greater than threshold Th1, unit SPU1 goes from non-detecting state NDET to detecting state DET and freezes the value of reference signal Sr,        when the value of signal Sd again becomes less than threshold Th1, unit SPU1 goes back into non-detecting state NDET and releases reference signal Sr, which is again dynamically generated by filtering of signal Sd.        
In the example shown in FIG. 2A, signal Sd begins to increase at an instant t0 and reaches threshold Th1 at an instant t1. Signal Sr does not vary perceptibly between t0 and t1 because it only copies the slow variations of signal Sd. Signal Sr is then locked (i.e., frozen) from instant t1 until an instant t2 where signal Sd again becomes less than Th1. After instant t2, signal Sr still does not vary perceptibly because it does not copy the falling edge of short duration of signal Sd.
In a variant of this method, sensor controller 10 supplies a signal Sd, the value of which decreases as object 12 approaches the sensor. Detection threshold Th1 is in this case chosen to be less than reference signal Sr and the detector goes into detecting state DET when the value of signal Sd becomes less than threshold Th1.
In such a method, the freezing of reference signal Sr prevents it from slowly approaching detection signal Sd, which would cause an undesirable return to the non-detecting state. Indeed, threshold Th1 would increase with signal Sr and detection signal Sd would find itself, at one time or another, less than threshold Th1.
Such a detecting method is satisfactory when the detecting time of an object is short. However, in certain applications where the object detecting time may be long, a modification of environmental parameters during the detection period of an object may lead to the detector becoming blocked.
FIGS. 3A, 3B illustrate this problem. The value of signal Sd begins to increase at an instant t0 and reaches threshold Th1 at an instant t1, causing the freezing of signal Sr (FIG. 3A). The detector goes from non-detecting state NDET to detecting state DET (FIG. 3B). At an instant t2, the environmental parameters change and cause a new increase of the signal Sd value, unrelated to a displacement of the object. At an instant t3, the object leaves the field of detection of the sensor or ceases to be in contact with the sensor. The value of signal Sd decreases to reach, at an instant t4, a lower value representative of the non-detecting state. This lower value is however greater than threshold Th1 due to the change of environmental parameters. The detector thus remains blocked in the detecting state.
This problem has been for example noticed in the following applications:
A proximity detector is integrated in the headphones of a digital music player. The detector allows the sound to be stopped automatically when the user is no longer wearing the headphones. It has been noted that the formation or deposition of humidity on the sensitive surface of the sensor, for example due to transpiration by the user, causes the value of detection signal Sd to increase. When the user removes the headphones, the value of signal Sd remains high, as shown in FIG. 3A, and the proximity detector remains in the detecting state.
A proximity detector is integrated in a mobile telephone equipped with a touch pad. During a telephone conversation, the detector is used to lock the touch pad and/or set the screen in low consumption mode when the user approaches the telephone to his ears. It has also been noted that the detector may be blocked in the detecting state after the deposition or the formation of humidity on the sensitive portion of the sensor. This may happen during a telephone conversation or due to the fact that the telephone has suddenly changed environments (for example after having been placed in a humid room such as a bathroom). In this case, the touchpad remains untimely locked, preventing the user from using the telephone.
It may therefore be desired to provide a method of detecting an object that is more resistant to variations of environmental parameters affecting the detection signal.